The present disclosure relates to exhaust sensors, and particularly to sensors with NOX reduction coatings.
The automotive industry has used exhaust gas sensors in automotive vehicles for many years to sense the composition of exhaust gases, namely, oxygen. For example, a sensor is used to determine the exhaust gas content for alteration and optimization of the air to fuel ratio (A/F) for combustion.
One type of sensor uses an ionically conductive solid electrolyte between porous electrodes. For oxygen, solid electrolyte sensors are used to measure oxygen activity differences between an unknown gas sample and a known gas sample. In the use of a sensor for automotive exhaust, the unknown gas is exhaust and the known gas, (i.e., reference gas), is usually atmospheric air because the oxygen content in air is relatively constant and readily accessible. This type of sensor is based on an electrochemical galvanic cell operating in a potentiometric mode to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force (xe2x80x9cemfxe2x80x9d) is developed between the electrodes according to the Nernst equation.
With the Nernst principle, chemical energy is converted into electromotive force. A gas sensor based upon this principle typically consists of an ionically conductive solid electrolyte material, a porous electrode with a porous protective overcoat exposed to exhaust gases (xe2x80x9cexhaust gas electrodexe2x80x9d), and a porous electrode exposed to a known gas"" partial pressure (xe2x80x9creference electrodexe2x80x9d). Sensors typically used in automotive applications use a yttrium stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of a particular gas, such as oxygen for example, that is present in an automobile engine""s exhaust. Also, a typical sensor has a ceramic heater attached to help maintain the sensor""s ionic conductivity. When opposite surfaces of the galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:       E    =                  (                              -            RT                                4            ⁢            F                          )            ⁢              ln        ⁡                  (                                    P                              O                2                            ref                                      P                              O                2                                              )                                where      ⁢              :              ⁢          xe2x80x83                                    E          =                      electromotive            ⁢                          xe2x80x83                        ⁢            force                                                        R          =                      universal            ⁢                          xe2x80x83                        ⁢            gas            ⁢                          xe2x80x83                        ⁢            constant                                                        F          =                      Faraday            ⁢                          xe2x80x83                        ⁢            constant                                                        T          =                      absolute            ⁢                          xe2x80x83                        ⁢            temperature            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            gas                                                                    P                          O              2                        ref                    =                      oxygen            ⁢                          xe2x80x83                        ⁢            partial            ⁢                          xe2x80x83                        ⁢            pressure            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            reference            ⁢                          xe2x80x83                        ⁢            gas                                                                    P                          O              2                                =                      oxygen            ⁢                          xe2x80x83                        ⁢            partial            ⁢                          xe2x80x83                        ⁢            pressure            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            exhaust            ⁢                          xe2x80x83                        ⁢            gas                              
Due to the large difference in oxygen partial pressure between fuel rich and fuel lean exhaust conditions, the electromotive force (emf) changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel-rich or fuel-lean, conditions without quantifying the actual air-to-fuel ratio of the exhaust mixture.
Oxygen sensors measure all of the oxygen present in the exhaust to make the correct determination when the oxygen content (air) exactly equals the hydrocarbon content (fuel). The oxygen sensor platinum electrode is very efficient at converting the hydrocarbons and carbon monoxide to form carbon dioxide and water. Unfortunately, the platinum electrode is unable to reduce the oxygen containing the species NOX, with hydrocarbons, to form nitrogen, carbon dioxide and water.
A fuel can have a hydrocarbon content such that it takes 14.70 parts air (oxygen) to exactly combust 1.00 part fuel to form the products of carbon dioxide and water. Some of the oxygen may also be consumed in the reaction of nitrogen and oxygen to form nitrous/nitric oxides. Similarly, some of the oxygen may be consumed in the reaction of sulfur and oxygen to form sulfuric/sulfurous oxides. Nitrous/nitric oxides or sulfurous/sulfuric oxides will not react with hydrocarbons on a platinum electrode. Therefore, hydrocarbons are present and the sensor does not switch at the correct point. As a result, excess air is introduced until the hydrocarbon is reacted and the oxygen is detected.
In this case, instead of switching at 14.70 parts air to 1.00-part fuel, the sensor switches at 14.75 parts air to 1.00 part fuel. This is referred to as a xe2x80x9clean shiftxe2x80x9d because the switching point is shifted more lean than the true stoichiometric point. Operating an engine with a lean shifted sensor results in excess nitrogen oxides being released to the atmosphere since there are not enough hydrocarbons to reduce the nitrogen oxides in the catalytic converter. Since nitrogen oxides are the exhaust component responsible for the formation of xe2x80x9csmogxe2x80x9d, it is desirable to correct the lean shift in the sensor.
One approach for treating nitrogen oxides in exhaust gases of engines operating under lean-burn conditions has been to incorporate NOX adsorbers in exhaust lines along with three way catalysts. Conventional exhaust systems contemplate any number of configurations, including for example, use of NOX adsorbers in the same canister along with three-way catalysts or use of a NOX adsorber in a separate can upstream of the three-way catalyst, among others. Rhodium is typically used in the industry to liberate oxygen from NOX. However, rhodium affects the sensor signal such that the platinum-rhodium electrodes provide no net improvement in sensor control and often an increase in the emissions will result.
A second approach has been to alter the oxygen sensor in an attempt to correct the lean shift. A platinum-rhodium mixture has been used to form the electrodes. However, such a composition depresses the sensor performance to an unacceptable level. Rhodium or a mixture of platinum and rhodium has also been used to form a separate layer in between the sensing electrode and protective layer. However, the durability of the rhodium and the functionality of the sensor are not as good as desired when such a layer is included.
With current sensor and catalyst technology, exhaust emissions are reduced about 99.8%. An increase in the catalyst quantity does not improve efficiency. One way to eliminate a significant portion of the last 0.2% pollutant is to correct for the lean shifted sensor. Accordingly, there remains a need in the art for lean shift corrected sensor technology.
The drawbacks and disadvantages of the prior art are overcome by a NOX reduction sensor coating.
A sensor is disclosed that comprises an electrolyte disposed between and in intimate contact with a sensing electrode and a reference electrode. A protective coating is disposed on the protective layer adjacent to the sensing electrode. The protective coating comprises a mixture of a metal oxide, a zeolite, and alumina.
A method for manufacturing a sensor is disclosed. The method comprises disposing an electrolyte between a sensing electrode and a reference electrode. A protective layer is disposed adjacent to the sensing electrode. A protective coating, which comprises a mixture of a metal oxide, a zeolite, and alumina is disposed in physical contact with the protective layer.